Although Chromecast Audio was discontinued, you can still make your vintage speakers or favourite AVR systems alive by using Chromecast to stream music to dumb speakers, just like what Chromecast Audio does.
Of course, this may require additional setup and one additional gadget (a few bucks) unless your speaker or AV supports HDMI input.
This Chromecast guide explains why you may consider using Chromecast to stream music to dumb speakers and how to set up the system so that you can continue streaming music to the speakers and AV systems without Chromecast Audio.
Update: the latest Chromecast 2020 (Chromecast with Google TV; Chromecast 4th Gen) merges Chromecast, Chromecast Ultra, and Android TV. There is no successor to Chromecast Audio.
Why do you want to use Chromecast (or Chromecast Ultra) as Chromecast Audio? Because Google discontinued Chromecast Audio!
In early 2019, Google silently discontinued Chromecast Audio. Now, it is almost impossible to get a “new” Chromecast Audio even on eBay.
The asking price of a “used” Chromecast Audio is about that of two new Chromecast units!